One-two punch from Del Potro. Another step ahead.

Nadal's lob flies over the head of Del Potro, who is scampering back into position. It's called long but Nadal challenges...

It is long!

Chanda Rubin
American former world number six at Flushing Meadows

How did that ball go over?!

Talk about luck!

Nadal whips out a backhand drop shot which clips the net and dribbles over the other side. No way of Del Potro getting that!

Del Potro regains control as Nadal goes high and long with his backhand.

Del Potro serves into the body but Nadal returns and sets up a rally. The Spaniard comes out on top with a fizzing cross-court forehand.

The angle on that... Oosh!

The Argentine consolidates the break with a forehand winner down the line.

Del Potro gets the break of serve immediately in the tie-break and wipes his forehead with relief.

Jonathan Jurejko
BBC Sport at Flushing Meadows

Sign of desperation from Delpo? He challenges his own serve after seeing a Nadal return whizz past him for 0-15. It was in.

That's his challenges down the drain and means he can't challenge a serve at 40-30. Doesn't matter in the end... he's over the line.

Chanda Rubin
American former world number six at Flushing Meadows

You have to feel going into this tie-break that Del Potro has a little bit of an advantage after holding the way he just did.

Tie-break here we come...

Del Potro's first serve is returned cross-court by Nadal. The Argentine challenges his own serve in the hope it's out so he can re-play the point. It's in.

Nadal is sensing an opportunity... he slides into position and fizzes a sublime, whipping forehand around the net. 30-15.

Del Potro responds well and goes 40-30 ahead with a thumping forehand winner down the line.

And he holds as Nadal returns into the net.

Jonathan Jurejko
BBC Sport at Flushing Meadows

Whenever a high ball comes back to Rafael Nadal, you know what is coming. I've barely seen him miss a smash. Eyes fixed on the ball, connection timed to perfection.

He's just turned the pressure up to gas mark five. Can he spark it on Delpo's serve?

NORZIELUS: I think that Del Potro will regret for a long time to have squandered two set points. Incredible!!!

Del Potro stretches to keep the ball in play and it's lofted high into the air but lands a metre inside the baseline. Nadal watches it carefully and slams an overhead volley down the line.

The defending champion repeats the process to go 40-0 up and holds for love with a backhand winner down the line.

He's ramping it up now...

Jonathan Jurejko
BBC Sport at Flushing Meadows

Well. You feel that could be a turning point in the set. Maybe the match. I know it is early, but...

Chanda Rubin
American former world number six at Flushing Meadows

Del Potro had that set on his racquet. He will be most disappointed about the forehands he missed. That is usually such a reliable shot for him.

Another error! Del Potro chokes. He smashes his backhand into the net and slams his racquet into his bag.

A new one is needed and he trudges back to the baseline. We're back on serve!

Del Potro opens up his shoulders to smash a forehand across court but he's wide again! Break point Nadal.

The Argentine bounces back to open up another set point but he squanders the chance again when his forehand smashes into the net.

Deuce again...